Correct a/c gauge readings?

Can someone tell me where to find the correct specs for low side/high side air conditioning gauge readings for particular vehicles.

I have a 1993 Mustang 2.3L and a 1997 Yukon 5.7L (with one a/c.)
I know it's in the 20-30 range on the low side, but is there an ideal number per each a/c system??

Thanks.